A LAN client can determine which router should be the first hop to a particular remote destination by using a dynamic process or static configuration. Examples of dynamic router discovery are as follows:
Proxy ARP—The client uses Address Resolution Protocol (ARP) to get the destination it wants to reach, and a router responds to the ARP request with its own MAC address.
Routing protocol—The client listens to dynamic routing protocol updates (for example, from Routing Information Protocol [RIP]) and forms its own routing table.
ICMP Router Discovery Protocol (IRDP) client—The client runs an Internet Control Message Protocol (ICMP) router discovery client.
The disadvantage to dynamic discovery protocols is that they incur some configuration and processing overhead on the LAN client. Also, if a router fails, the process of switching to another router can be slow.
An alternative to dynamic discovery protocols is to statically configure a default router on the client. Although, this approach simplifies client configuration and processing, it creates a single point of failure. If the default gateway fails, the LAN client is limited to communicating only on the local IP network segment and is cut off from the rest of the network.
VRRP can solve the static configuration problem by enabling a group of routers (a VRRP group) to share a single virtual IP address.
You can then configure the LAN clients with the virtual IP address as their default gateway.
Figure 1. Basic VRRP Topology. This image shows a basic VLAN topology where routers A, B, and C form a VRRP group. The IP address of the VRRP group must be different to the address that was configured for the Ethernet interface of Router A (10.0.0.1).
The VRRP IP address must be different to any physical IP address of the devices participating in the VRRP,
otherwise the ARP or MAC entries will be corrupted and may cause
Because the virtual IP address uses the IP address of the physical Ethernet interface of Router A, Router A is the master (also known as the IP address owner). As the master, Router A owns the virtual IP address of the VRRP group and forwards packets sent to this IP address. Clients 1 through 3 are configured with the default gateway IP address of 10.0.0.1.
Routers B and C function as backups. If the master fails, the backup router with the highest priority becomes the master and takes over the virtual IP address to provide uninterrupted service for the LAN hosts. When router A recovers, it becomes the master again.
In Cisco NX-OS Release 4.1(2) and later, packets received on a routed port destined for the VRRP virtual IP address terminates on the local router, regardless of whether that router is the master VRRP router or a backup VRRP router. This includes ping and Telnet traffic. Packets received on a Layer 2 (VLAN) interface destined for the VRRP virtual IP address terminates on the master router.